This invention relates to improved erosion-resistant coatings for metal objects which are used under conditions of air turbulence, high stress, and high temperatures. More specifically, the invention concerns the coating of aircraft turbine compressor blades and stators with a composite coating having two phases, the combined properties of which lead to dramatic increases in resistance to erosion caused by impacting particulate matter.
Over the past 20 years, there has been a nearly complete conversion of commercial and military aircraft to the use of turbine engines for propulsion. This has resulted in an increase in mechanical reliability and in increased efficiency, particularly at high altitudes. However, a number of other problems are faced by turbine engines, including blade wear in the compressor section due to the impact of particulate matter.
In helicopter applications this problem is intensified by the unique erosion problems engendered by the self-induced air turbulence associated with take-off, landing, and hovering near the ground. Small gas turbines powering helicopters, hovercraft, etc., over dusty, unimproved land areas routinely ingest up to 25 pounds of sand and dust every hour of low altitude operation.
The abrasive particles impact the critical air-foil surfaces of compressor blades and vanes, eroding the thin metal tips and leading edges. This results in rapid progressive deterioration of engine performance.
One approach to a solution for this problem is a filtration system to remove dust and sand from the air stream. However, dust filters and separators invariably decrease engine efficiency and require constant maintenance.
A second approach is to select blade and vane materials with improved inherent erosion resistance. Even still, high strength metallic materials, including the 12-17 percent chromium-stainless steel and titanium alloys which are used for current compressor blades and vanes, do not possess adequate intrinsic resistance to dust erosion. In addition, it has been found that the use of even harder materials in blade construction leads to an undesirable loss of ductility and impact resistance.
Another approach has been to coat the standard blade materials with an erosion-resistant outerlayer. The difficulty here arises in obtaining good adhesion. For this reason, the patent literature reveals that a number of diffusion coating techniques, multi-layer coating techniques, etc., have been devised. For example in U.S. Pat. No. 3,594,219 to Maxwell, it is disclosed that a nickel or cobalt-base superalloy undercoating may be used as an adherent base for aluminide overcoatings. Likewise, Darnell in U.S. Pat. No. 3,368,914 suggests that one means of obtaining a better coating is to diffuse-coat a metal carbide onto the steel substrate and then bond a thicker metal carbide coat over that. Similarly, in U.S. Pat. No. 3,772,058 to Bloom there is disclosed a vapor deposition method for applying a hard ceramic layer of titanium carbonitride over a nickel layer.
While such ceramic-type coatings are more erosion-resistant, these materials are brittle and lead to reduced fatigue life. For example, tests on coatings such as Ti-Kote-C, a titanium carbonitride (TIC.sub.0.5 N.sub.0.5) solution of Texas Instruments, showed a 30-50 percent loss in airfoil fatigue strength.
Apparently as a partial solution for this problem, it has been proposed to use a more ductile nickel or cobalt layer beneath the hard outer layer. Additional examples of this are found in U.S. Pat. No. 2,767,464 to Nack wherein there is used a nickel layer having a slight tendency to yield and take up the effects of applied forces to relieve the strains in the more brittle chromium layer used as the outer coating. Similarly, Geotzel in U.S. Pat. No. 2,612,442 utilizes a nickel or cobalt intermediate layer between the steel blade and the hard titanium carbide outer layer.
Even still, the dual coatings disclosed in these patents, while an improvement in erosion-resistance over the bare blade, do not go as far as desired in solving the problem. As should be apparent every increase in erosion resistance gives added operating hours to aircraft use, leading to considerable economic savings in repair and replacement costs. Turbine blade erosion resistance remains, therefore, a major objective in the aircraft industry.
The mechanism by which improvement in erosion resistance is obtained is not well understood. Hardness, i.e., scratch resistance, is certainly a factor in decreasing local erosion on a microscale. However, the impact resistance of the material is important as it relates to the transmission of the shock wave from the impacting particles, the resultant stresses set up within the material, and the reaction of the coating to these stresses. A common example of failure by such stresses is the expulsion of material from a crater in the direction opposite to that of impact when plate glass is penetrated by a bullet. These first two factors relate to the wear resistance of monolithic materials. When such materials are applied as coatings to dissimilar materials such as those used in compressor blades, additional factors come into play. For thin but continuous coatings containing zero residual stress, when the modulus of elasticity of the substrate is appreciably less than that of the coating, the coating may not be adequately supported on impact, and it may fail by flexure beyond its elastic limit. In continuous coatings, adhesion of the coating to the substrate is important in that impact of a particle generates a reflected tensile stress at the interface which may result in separation of the coating and lack of support for subsequent impacts.
In cases where the coefficient of thermal expansion of the coating is higher than that of the substrate, tensile stresses are set up in the coating on cooling which tend to crack the coating. In cases where the coefficient of expansion of the coating is less than that of the substrate, and the coating is adherent, a pattern of cracks with loss of material along the cracks can result from compressive stress.
Where the coating is crazed, i.e., separated islands of coating material in a reticule of open cracks, and where the coating adhesion is good, this does not necessarily obviate the value of the coating. However, the stress pattern generated by impact near a crack is more deleterious than that generated by impact in the center of one of these coating islands, and the erosion resistance of such a coating is strongly dependent on the adhesion of the coating material to the substrate.
When adhesion of the coating material to the substrate is particularly good, the erosion resistance may be characterized by the further cracking of the coating into fragments.
Obviously when the coating is penetrated in any of the above mechanisms, subsequent erosion is that characteristic of the substrate material and ultimate failure of the blade system results.
Another factor in the effectiveness of the coatings is their ability to prevent crack propagation from the outer portion of the coating into the substrate. For example, it has been shown that a coating of brittle tungsten carbide directly on the blades of concern to this invention underwent premature fatigue failure, presumably because of crack propagation. The presence of the low-modulus relatively ductile nickel in the coating may help prevent this type of failure, but even then tungsten carbide over nickel does not give the desired increases in erosion resistance.
Overcoming or minimizing all of these factors is necessary to achieve an erosion resistant coating which is superior to those previously known.